We report the distribution of tyrosine hydroxylase immunoreactive (TH-ir) neurons in the brain of adult and developing trout. In the adult brain TH-ir neurons were observed in the telencephalon (in the olfactory bulbs and the ventral telencephalic region), the diencephalon (in both the parvicellular and magnocellular portions of the preoptic nucleus, the suprachiasmatic nucleus, the ventromedial thalamic nucleus, the posterior tuberal nucleus, the organon vasculosum hypothalami, the lateral recess nucleus and the dorsal periventricular region of the medial lobe recess), the pretectal region (nucleus pretectalis periventricularis), the isthmal region (locus coeruleus) and the medulla oblongata (in the area postrema, nucleus solitarius, the reticular area of the vagal region, and the reticular nucleus of the octaval region). The earliest TH-ir nuclei were observed in 10- to 11-min embryos in the posterior tuberal nucleus, locus coeruleus, nucleus solitarius and vagal reticular neurons. In the olfactory bulb, nucleus ventromedialis thalami and octaval reticular area, TH-ir cells a latter appeared at a later stage in development. In fry, juveniles and adults cerebrospinal fluid-contacting TH-ir cells were observed in circumventricular organs. The evolution of these TH-ir neuronal populations during development is described, and we discuss their significance for both comparative and developmental analysis.